1. Field of the Invention
The present invention relates to production of a low-magnetic cast iron of which matrix structure principally consists of austenite by containing a proper balance of austenite-stabilizing elements, manganese (Mn) and carbon (C), in its cast iron composition. In addition, it relates to a spheroidal graphite cast iron which has quality of material with excellent wear resistance by high work hardening ability caused by containing a large amount of Mn, and with suppressed strength reduction because of existence of graphite in the matrix structure by making the shape of graphite spheroidal being the characteristic of this cast iron and dispersed in the matrix structure, and stress concentration around graphite reduced. Furthermore, it relates to a nonmagnetized and toughened spheroidal graphite cast iron by heat treatment suitable for the cast iron according to the present invention.
2. Description of the Related Art
Austenitic spheroidal graphite cast irons and high-manganese steels are non-magnetic or low magnetic, since their matrix structures are mainly austenite and the austenite is non-magnetic. Ferromagnetic materials with high magnetic permeability have problems that heat is produced, large energy loss is produced, and the material itself is heated by eddy current induced from electromagnetic induction around strong electromagnets. As structural materials for such a condition, ductile Ni-resist cast irons and austenitic stainless-steels are used.
Spheroidal graphite cast irons are materials although being a type of cast irons with very small influence on strength reduction by graphite dispersed in the matrix structure, since, just as their name says, the shape of graphite is spheroidal. In addition, they have better castability when compared with steels, and have excellent machinability for their hardness. Among austenitic spheroidal graphite cast irons, ductile Ni-resist cast irons are widely known, and are used for such as exhaust manifolds, vacuum pumps, and working machines that require thermal-stability, corrosion resistance, and small thermal expansion coefficient. Also, they are low magnetic and have excellent low-temperature toughness, since their matrix structures are austenite. However, since they comprise 12-36 weight % of Ni, their cost as structural materials for low-magnetism and low-temperature toughness is too high. In addition, a problem is left as structural materials that, since they comprise a large amount of Ni, their proof strength and tensile strength are moderate as 210-310 MPa and 370-500 MPa, respectively. As for an austenitic spheroidal graphite cast iron in which a part of Ni was substituted with Mn, it is shown in Prior Art Document, JP Unexamined Patent Publication No. 2004-218027 (Patent Document 1).
High-manganese steels are a kind of materials invented by an English inventor, Robert Abott Hadfield, in 1882, and most commonly comprise 1 wt. % of C and 13 wt. % of Mn. They are unique materials, since they have high strength, toughness, and high work hardening ability, only processed surface is hardened and have higher wear resistance, while not processed inner parts is soft and have high toughness. Usually, excellent quality of materials described above is realized by heat treatment. Since their matrix structure is austenite, they are non-magnetic, and they have toughness even at the liquid nitrogen temperature (77K) since austenite is stable, and their application in low-temperature environments is becoming larger. Again in recent years, they have received attention as structural materials around nuclear fusion reactors and superconductivity systems, and as structural materials for such as guide way of linear motor cars and liquefied gas storage tanks. However, high-manganese steels have very low machinability, and although their cost is lower when compared with other austenitic steels, their demand expansion is limited. Furthermore, when compared with cast irons, their melting point is higher and their castability is worse, and have problems that it is very difficult to manufacture products with complex and thin shapes.    Patent Document 1: JP Unexamined Patent Publication No. 2004-218027